With the increasing controversy surrounding fossil fuel based emissions, significant resources are being devoted to developing alternative and renewable energies. Energy production from salinity gradients has been known for several decades and offers significant advantages of carbonless emissions and renewability. Two leading methods of energy production from salinity gradients are pressure retarded osmosis (PRO) and reverse electrodialysis (RED). Reverse electrodialysis functions on the basis of salinity differences in mixing solution such as sea water and river water. A membrane stack is sandwiched by electrodes and composed of alternating salt water and freshwater compartments defined by anion and cation exchange membranes permitting selective exchange of ions between the compartments. Driven by the difference in chemical potential between the salt water and freshwater solutions, cations diffuse through the cation exchange membranes toward the cathode and anions diffuse through the anion exchange membranes toward the anode. At the electrodes, a redox couple is used to mitigate the transfer of electrons. Therefore, when electrodes are connected to an external circuit, electrical power can be extracted from the reverse electrodialysis system.
The theoretical power density of reverse electrodialysis systems is in the neighborhood of 20 W/m2. In actual implementation, however, the power density drops significantly to 1-3 W/m2 in the most efficient systems. The precipitous drop in power density can be attributed to several factors including resistances through the cation and anion exchange membranes, electrode resistance and power consumed in plant operation. In order to compensate for these losses, significant membrane area is required. This can be prohibitive given the high cost and limited lifetimes of suitable cation and anion exchange membranes.